ESR and ENDOR Study of Single Crystals of Deoxyadenosine Monohydrate X- Irradiated at 10 K

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Five free radicals have been detected by detailed ESR/ENDOR experiments on single crystals of deoxyadenosine monohydrate (AdRm), X-irradiated and observed at 10 K. In a previous study of adenosine (Radiat. Res. 117, 367- 378, 1989), only the anion (protonated at N3) and the cation (deprotonated at the exocyclic NH2) were detected at 10 K. In AdRm, Radical R1 is the N3- protonated anion, similar to that observed previously in adenosine. Radical R3 is a C5' hydroxyalkyl radical formed by net H-abstraction from C5'. A second sugar radical is formed by net C1' H-abstraction. Two other base radicals observed in AdRm at 10 K are the C2 and C8 H-addition radicals. The C2 H-addition radical (Radical R5) exhibits inequivalent methylene hydrogen couplings of 5.43 and 3.29 mT, while in the C8 H-addition radical (Radical R6) the couplings are somewhat more equivalent (3.63 and 4.17 mT). No link between Radical R1 and the H-addition radicals has been observed. The reduced base appears to protonate rapidly even at 10 K, while at the same time both H-addition radicals are clearly present. On warming, Radical R1 appears to decay at about 80 K with no apparent successor. Although no base cation was stabilized in AdRm at 10 K, it is interesting to note that Radicals R3 and R4 could both be formed as the result of deprotonation of primary oxidation products.